I. Characteristics of Sleep Apnea
First described in 1965, sleep apnea is a breathing disorder characterized by brief interruptions (10 seconds or more) of breathing during sleep. Sleep apnea is a common but serious, potentially life-threatening condition, affecting as many as 18 million Americans. Snoring also can occur independent of or during a sleep apneic event.
There are two types of sleep apnea: central and obstructive. Central sleep apnea, occurs when the brain fails to send the appropriate signal to the breathing muscles to initiate respirations, e.g., as a result of brain stem injury or damage. Mechanical ventilation is the only treatment available to ensure continued breathing.
Obstructive sleep apnea (OSA) is far more common. Normally, the muscles of the upper part of the throat keep the airway open to permit air flow into the lungs. When the muscles at the base of the tongue and the uvula (the small fleshy tissue hanging from the center of the back of the throat) relax and sag, the relaxed tissues may vibrate as air flows past the tissues during breathing, resulting in snoring. Snoring affects about half of men and 25 percent of women—most of whom are age 50 or older.
In more serious cases, the airway becomes blocked, making breathing labored and noisy, or even stopping it altogether. In a given night, the number of involuntary breathing pauses or “apneic events” may be as high as 20 to 30 or more per hour. These breathing pauses are almost always accompanied by snoring between apnea episodes, although not everyone who snores has the condition. Sleep apnea can also be characterized by choking sensations.
Lack of air intake into the lungs results in lower levels of oxygen and increased levels of carbon dioxide in the blood. The altered levels of oxygen and carbon dioxide alert the brain to resume breathing and cause arousal. The frequent interruptions of deep, restorative sleep often lead to early morning headaches, excessive daytime sleepiness, depression, irritability, and learning and memory difficulties.
The medical community has become aware of the increased incidence of heart attacks, hypertension and strokes in people with moderate or severe obstructive sleep apnea. It is estimated that up to 50 percent of sleep apnea patients have high blood pressure.
Upon an apneic event, the sleeping person is unable to continue normal respiratory function and the level of oxygen saturation in the blood is reduced. The brain will sense the condition and cause the sleeper to struggle and gasp for air. Breathing will then resume, often followed by continued apneic events. There are potentially damaging effects to the heart and blood vessels due to abrupt compensatory swings in blood pressure. Upon each event, the sleeping person will be partially aroused from sleep, resulting in a greatly reduced quality of sleep and associated daytime fatigue.
Although some apneic events are normal in all persons and mammals, the frequency of blockages will determine the seriousness of the disease and opportunity for health damage. When the incidence of blockage is frequent, corrective action should be taken.
II. The Anatomy of the Upper Airway
As FIG. 1 shows, the upper airway consists of a conduit that begins at the nasal valve, situated in the tip of the nose, and extends to the larynx, which is also called the voice box because it houses the vocal cords. The pharynx (which, in Greek, means “throat”) is a cone-shaped passageway in the upper airway that leads from the oral and nasal cavities in the head to the esophagus and larynx. The pharynx serves both respiratory and digestive functions. Both circular and longitudinal muscles are present in the walls of this organ, which are called the pharyngeal walls. The circular muscles form constrictions that help push food to the esophagus and prevent air from being swallowed, while the longitudinal muscles lift the walls of the pharynx during swallowing.
The pharynx consists of three main divisions. The superior portion is the nasal pharynx, the back section of the nasal cavity. The nasal pharynx connects to the second region, the oral pharynx, by means of a passage called an isthmus. The oral pharynx begins at the back of the mouth cavity and continues down the throat to the epiglottis, a flap of tissue that covers the air passage to the lungs and that channels food to the esophagus. The isthmus connecting the oral and nasal regions allows humans to breathe through either the nose or the mouth. The third region is the laryngeal pharynx, which begins at the epiglottis and leads down to the esophagus. Its function is to regulate the passage of air to the lungs and food to the esophagus. Air from the nasal cavity flows into the larynx, and food from the oral cavity is routed to the esophagus directly behind the larynx. The epiglottis, a cartilaginous, leaf-shaped flap, functions as a lid to the larynx and, during the act of swallowing, controls the traffic of air and food.
The mouth cavity marks the start of the digestive tube. Oval in shape, it consists of two parts: the vestibule and the mouth cavity proper.
The vestibule is the smaller outer portion, delimited externally by the lips and cheeks and internally by the gums and teeth. It connects with the body surface through the rima or orifice of the mouth. The vestibule receives the secretion of the parotid salivary glands and connects when the jaws are closed with the mouth cavity proper by an aperture on both sides behind the wisdom teeth, and by narrow clefts between opposing teeth.
The mouth cavity proper contains the tongue and is delimited laterally and in the front by the alveolar arches with the teeth therein contained. The alveolar process on the upper jaw is contained in the maxillae, whereas the alveolar process on the lower jaw is contained in the mandible. The mandible is a U-shaped bone that supports the mandibular (lower) teeth.
The mouth cavity proper receives the secretion from the submaxillary and sublingual salivary glands. The mouth cavity proper connects with the pharynx by a constricted aperture called isthmus faucium.
The tongue (see FIG. 1B) is a mobile muscular organ that can assume a variety of shapes and positions. The tongue comprises extrinsic and intrinsic muscles. The extrinsic muscles (genioglossus, hyoglossus, styloglossus, and palatoglossus) (shown in FIG. 1B) have their origin in other structures and attach to the tongue. Their function is to move the tongue and, at times, change its shape. The intrinsic muscles of the tongue (superior longitudinal, inferior longitudinal, transverse, vertical) (not shown with particularity) are attached entirely within the tongue work to modify the shape of the tongue. The inferior surface of the tongue (see FIG. 1C) is covered with a thin, transparent mucous membrane through which one can see the underlying veins. With the tongue raised (as shown in FIG. 1C), the lingual frenulum is exposed. The lingual frenulum is a large, midline fold of mucosa that connects the tongue to the floor of the mouth, while allowing the anterior part of the tongue to move freely.
The tongue has a relatively fixed inferior part that is attached to the hyoid bone and mandible. The rest of the tongue is called the body of the tongue. It is essentially a mass of muscles (that is mostly covered by mucous membrane. The muscles in the tongue do not act in isolation. Some muscles perform multiple actions with parts of one muscle acting independently producing different, sometimes antagonistic, actions.
The tongue is partly in the mouth or oral cavity and partly in the pharynx. At rest, it occupies essentially the entire oral cavity. The posterior part of the tongue demarcates the posterior boundary of the oral cavity. Its mucous membrane is thick and freely movable.
The tongue is involved with mastication, taste, articulation, and oral cleansing. Its two main functions are forming words during speaking and squeezing food into the pharynx when swallowing.
The epiglottis is a protective fold of the cartilage posterior to the base of the tongue and in front of the larynx. When a human breathes, the epiglottis stands up, allowing air to go into the larynx and lungs. During swallowing, the epiglottis folds back to cover the larynx and keep food from entering the windpipe and lungs. Once the swallowing is over, the epiglottis resumes its upright position.
The palate forms the arched roof of the oral or mouth cavity (the mouth) and the floor of the nasal cavities (the nose). It separates the oral cavity from the nasal cavities and the nasal pharynx. The palate consists of two regions—the hard palate anteriorly and the soft palate posteriorly.
The hard palate is vaulted and defines the space filled by the tongue when it is at rest. The hard palate is bounded in the front and laterally by the alveolar arches and gums and in the back by the soft palate. A dense structure made up by the periosteum and the mucous membrane of the mouth covers the hard palate. The linear raphe lies along the middle line of the hard palate. The hard palate has a hard bony skeleton, hence its name.
The soft palate has no bony skeleton, hence its name. The soft palate is a movable fold, suspended from the posterior border of the hard palate and forms an incomplete dividing line (septum) between the mouth and the pharynx. The soft palate comprises a mucous membrane that envelops muscular fibers, an aponeurosis, vessels, nerves, adenoid tissue, and mucous glands. When the soft palate is relaxed and hanging, the anterior surface is concave and follows the same line as the roof of the mouth. The posterior surface of the soft palate is convex and is a continuance of the mucous membrane that covers the bottom part of the nasal cavities. The upper boundary of the soft palate attaches to the hard palate; the sides become part of the pharynx; and the lower boundary is free. The lower boundary which hangs down, separating the mouth and the pharynx is known as the palatine velum. In the middle of the lower boundary, the small, fleshy cone-shaped protuberance is called the uvula; the uvula prevents the food from entering the nasopharynx and the muscles of the soft palate push the food down into the pharynx. The arches are located laterally and downwardly from the uvula. These arches are called the glossopalatine arch (the anterior arch) and the pharyngopalatine arch (the posterior arch). The palatine aponeurosis is a thin, firm fiber-filled lamella which gives support to the muscles and makes the soft palate strong.
The soft palate is suspended from the posterior border of the hard palate. It extends posteriorly and inferiorly as a curved free margin from which hangs a conical process, called the uvula; closely following behind the soft palate are the palatoglossal and the palatopharyngeal arches, respectively. Muscles arise from the base of the cranium and descend into the soft palate. The muscles allow the soft palate to be elevated during swallowing into contact with the posterior pharyngeal wall. The muscles also allow the soft palate to be drawn inferiorly during swallowing into contact with the posterior part of the tongue.
The soft palate is thereby very dynamic and movable. When a person swallows, the soft palate initially is tensed to allow the tongue to press against it, to squeeze the bolus of food to the back of the mouth. The soft palate is then elevated posteriorly and superiorly against the pharyngeal wall, acting as a valve to prevent passage of food into the nasal cavity.
Caudal to the soft palate, the hyoid bone is situated at the base of the tongue in the anterior part of the neck at the level of the C3 vertebra and in the angle between the mandible and the thyroid cartilage of the larynx, the voice box. It is a symmetric U-shaped bone (see FIG. 2B), comprising a body with greater horns and lesser horns, which serve as points of attachment for numerous muscles in the tongue, pharynx, and the anterolateral part of the neck (see FIGS. 3A to 3D).
The hyoid bone does not articulate with any other bone. It serves a purely anchoring function for muscles. The hyoid bone is suspended from the styloid processes of the temporal bones by the stylohyoid ligaments and is firmly bound to the thyroid cartilage. Functionally, the hyoid bone serves as an attachment point for numerous muscles and a prop to keep the airway open. The primary function of the hyoid bone is to serve as an anchoring structure for the tongue.
FIGS. 3A to 3D show some of the numerous muscles that are attached to the hyoid bone (as does FIG. 1B). The muscles attached to the hyoid bone include the middle pharyngeal constrictor muscle (see FIG. 3A), which attaches at the end of the greater horns. The middle pharyngeal constrictor muscle, together with the superior and inferior pharyngeal constrictor muscles (also shown in FIG. 3A), extend along the upper airway. As before stated, a change in muscle function of the pharyngeal constrictor muscles can lead to pharyngeal narrowing and collapse.
The muscles attached to the hyoid bone also include the hyoglossus muscles (see FIGS. 3B and 3D, as well as FIG. 1B). The hyoglossus muscles originate along the entire length of each greater horn and also from the body of the hyoid. The hyoglossus muscles are inserted into the posterior half or more of the sides of the tongue, as FIG. 3D best shows. The hyoid bone anchors the hyoglossus muscles when they contract, to depress the tongue and to widen the oral cavity, thereby opening the airway.
The muscles attached to the hyoid bone also include the two geniohyoid muscles (see FIG. 3C). The geniohyoid muscles originate close to the point at which the two halves of the lower jaw meet; the fibers of the muscles extend downward and backward, close to the central line, to be inserted into the body of the hyoid bone. Contraction of the geniohyoid muscles pulls the hyoid bone upward and forward, shortening the floor of the mouth and widening the pharynx.
Inserting into the middle part of the lower border of the hyoid bone are the sternohyoids (see FIG. 3C). The sternohyoids are long muscles arising from the breastbone and collarbone and running upward and toward each other in the neck. The sternohyoids depress the hyoid bone after it has been elevated during swallowing.
Other muscles attached to the hyoid bone are the two mylohyoid muscles (see FIG. 3C), which form a sort of diaphragm for the floor of the mouth, elevating the floor of the mouth and tongue during swallowing; the thyrohyoid (see FIG. 3C), arising from the thyroid cartilage of the larynx, which elevates the larynx; and the omohyoid (see FIG. 3C), which originates from the upper margin of the shoulder blade, which depresses, retracts, and steadies the hyoid bone.
The position of the hyoid bone with relation to the muscles attached to it has been likened to that of a ship steadied as it rides when anchored “fore and aft.” Through the muscle attachments, the hyoid plays an important role in mastication, in swallowing, and in voice production.
The larynx, also known as the organ of voice, is part of the upper respiratory tract. As FIG. 1A shows, the larynx is situated between the base of the tongue and the trachea; vertically, the larynx's position corresponds to the C4, C5, and C6 vertebrae, although this location is higher in females and during childhood. FIG. 2A shows the nine cartilages of the larynx: a thyroid, a cricoid, two arytenoids, two corniculate, two cuneiform, and an epiglottis.
The larynx comprises extrinsic ligaments which link the thyroid cartilage and the epiglottis with the hyoid bone and the cricoid cartilage with the trachea (see FIG. 2B). The hyothyroid membrane and the lateral hyothyroid ligament attach the thyroid cartilage to the hyoid bone. The hyoepiglottic ligament connects the epiglottis to the upper border of the hyoid bone. The cricotracheal ligament attaches the cricoid cartilage to the first ring of the trachea (see FIG. 2B).
III. Sleep and the Anatomy of the Upper Airway
Although all tissue along this conduit is dynamic and responsive to the respiratory cycle, only the pharynx is totally collapsible. The pharyngeal structures and individual anatomic components within this region include the pharyngeal walls, the base of the tongue, the soft palate with uvula, and the epiglottis.
The cross-sectional area of the upper airway varies with the phases of the respiratory cycle. At the initiation of inspiration (Phase I), the airway begins to dilate and then to remain relatively constant through the remainder of inspiration (Phase II). At the onset of expiration (Phase III) the airway begins to enlarge, reaching maximum diameter and then diminishing in size so that at the end of expiration (Phase IV), it is at its narrowest, corresponding to the time when the upper airway dilator muscles are least active, and positive intraluminal pressure is lowest. The upper airway, therefore, has the greatest potential for collapse and closure at end-expiration [ref: Schwab R J, Goldberg A N. Upper airway assessment: radiographic and other imaging techniques. Otolaryngol Clin North Am 1998, 31:931-968].
Sleep is characterized by a reduction in upper airway dilator muscle activity. For the individual who snores or has obstructive sleep apnea (OSA) and perhaps the other disorders which comprise much of the group of entities called obstructive sleep-disordered breathing (SDB), it is believed that this change in muscle function causes pharyngeal narrowing and collapse. Two possible etiologies for this phenomenon in OSA patients have been theorized. One is that these individuals reduce the airway dilator muscle tone more than non-apneics during sleep (the neural theory). The other is that all individuals experience the same reduction in dilator activity in sleep, but that the apneic has a pharynx that is structurally less stable (the anatomic theory). Both theories may in fact be contributors to OSA, but current studies seem to support that OSA patients have an intrinsically structurally narrowed and more collapsible pharynx. [Ref: Isono S. Remmers J, Tanaka A Sho Y, Sato J, Nishino T. Anatomy of pharynx in patients with obstructive sleep apnea and in normal subjects. J Appl Physiol 1997:82:1319-1326.] Although this phenomenon is often accentuated at specific sites, such as the velopharyngeal level [Isono], studies of closing pressures [Isono] support dynamic fast MRI imaging that shows narrowing and collapse usually occurs along the entire length of the pharynx. [Ref: Shellock F G, Schatz C J, Julien P, Silverman J M, Steinberg F, Foo T K F, Hopp M L, Westbrook P R. Occlusion and narrowing of the pharyngeal airway in obstructive sleep apnea: evaluation by ultrafast spoiled GRASS MR imaging. Am J of Roentgenology 1992:158:1019-1024].
IV. Treatment Options
To date, the only treatment modality that addresses collapse along the entire upper airway is mechanical positive pressure breathing devices, such as continuous positive airway pressure (CPAP) machines. All other modalities, such as various surgical procedures and oral appliances, by their nature, address specific sectors of the airway (such as palate, tongue base and hyoid levels), but leave portions of pharyngeal wall untreated. This may account for the considerably higher success rate of CPAP over surgery and appliances in controlling OSA. Although CPAP, which in essence acts as an airway splint for the respiratory cycle, is highly successful, it has some very significant shortcomings. It can be cumbersome to wear and travel with, difficult to accept on a social level, and not tolerated by many (for reasons such as claustrophobia, facial and nasal mask pressure sores, airway irritation). These factors have lead to a relatively poor long-term compliance rate. One study has shown that 65% of patients abandon their CPAP treatment in 6 months. Other current treatments for OSA include genioglossal advancement (GA), maxillomandibular advancement (MA), and hyoid myotomy. InfluENT Medical offers a genioglossus advancement procedure where suture loop is passed through the tongue and anchored to a screw essentially inserted into the mandible. In another procedure, hyoid myotomy and suspension, the hyoid bone is advanced using a suture tied to the hyoid bone anchors the structure to two screws placed in the mandible. These treatments involve highly invasive surgical procedures and a long recovery time, and therefore have relatively low patient appeal.
The need remains for simple, minimally invasive, cost-effective devices, systems, and methods for reducing or preventing sleep disordered breathing events.